The present invention relates more particularly to improvements in the foregoing processes, in general termed hydrothermal processes, for removing undesired constituents, especially sulfur, ash, or both, from a solid carbonaceous fuel of the coal or coke type. The coal, coke or the like is ground into fine particles that are mixed with an aqueous leaching solution and subjected to a pressure leaching operation.
The pressure leaching is performed at elevated temperature and pressure for a period of time sufficient to dissolve the undesired constituents to a desired extent. Thereby the undesired constituent content of the fuel is reduced to a value below an acceptable limit. This limit is usually dictated by environmental protection standards.
The solid fuel particles are then separated from the leaching solution, which carries away the undesired constituents removed during the leaching. The product is a clean solid fuel that can be more readily burned, liquefied, gasified, or otherwise utilized. It causes considerably less fouling and damage to equipment, and significantly less pollution of the environment, than the original fuel.
Pressure leaching has been employed in the metallurgical industry for separation of metallic components by the selective solubilization of individual compounds. This is achieved by heating an ore concentrate or a mixture of the metal components in an aqueous solution, acidic or basic. Selectivity, i.e., selective solubilization of the components, is achieved by adjusting the reaction parameters -- temperature, pressure, time, pH of the solution, and type of leachant. For example, FeO separation from TiO.sub.2 in ilmenite ore (FeO -- TiO.sub.2) is achieved by pressure leaching of the ore in sulfuric acid at elevated temperatures and pressures. Another example, which illustrates the behavior of metal compounds in alkali solutions, is the extraction of aluminia from bauxite ores. In this case, the ore is heated in sodium hydroxide solution at elevated temperature and pressures to selectively solubilize the aluminum value. The solution, containing the solubilized aluminum after separation from the insoluble portion of the ore, is then cooled, whereupon the aluminum values precipitate as aluminum hydroxide. If the solution containing the aluminum values were cooled and let stand in the presence of the insoluble portion of the ore, precipitation of the aluminum hydrate onto the insoluble portion of the ore would occur.
In the hydrothermal treatment of coal, it has been discovered that similar solubilization and precipitation processes must be appropriately managed. A significant portion of the ash and the sulfur are solubilized by pressure leaching of the coal in aqueous solutions. Cooling of these aqueous solutions in the presence of the clean coal can result in contamination of the coal by ash precipitated from solution onto the coal. However, this contamination can be prevented by separating the solution containing the ash and sulfur from the solution before precipitation of the ash can occur. One specific method for achieving separation of solubilized ash from the clean coal is pressure filtration. Pressure filtration achieves another goal, that is, it prevents contamination of the clean coal by the reprecipitation of a portion of the solubilized sulfur. Another advantage is that it allows rapid separation of the clean coal from the aqueous solution. If the solution is allowed to cool in the presence of the coal, a finely divided precipitate forms. This precipitate significantly reduces the rate of filtration, i.e., the separation of the clean coal from the spent leach liquor.
The present invention is concerned with precipitation, adsorption and chemical recombination effects which differ significantly from the mere solidification or freezing of elemental sulfur that occurs on cooling of a hot water slurry in other coal desulfurization processes such as that described in U.S. Pat. No. 3,824,084 to Dillon, wherein the slurry is filtered at temperatures above the freeze point of sulfur. The present invention contemplates the use of leaching solutions such as a sodium hydroxide solution in which elemental sulfur cannot exist.
It has been found that the separation of the spent leachant from the product slurry, obtained by the hydrothermal leaching treatment, at the temperature and the pressure of treatment results in a lower sodium and ash content of the solid fuel product than that resulting from the conventional processing of the product slurry. One exploratory experiment was performed to determine if the sodium and the ash content could be lowered further by carrying out the pressure filtration at temperatures other than the leaching temperature. In the experiment a coal was treated with NaOH at 250.degree. C, the product slurry was allowed to cool to 200.degree. C and then pressure filtration was started. The rate of filtration was found to be extremely slow, indicating that the frit (used for filtration) was plugged. However, on reheating of the slurry to 250.degree. C the rate of filtration was greatly improved.
The above results suggest that the cooling of the product slurry in the precipitation of the ash dissolved during hydrothermal treatment. Moreover, the process of precipitation on cooling and dissolution on heating is a reversible one.
Additional experiments were carried out using high-temperature, high-pressure filtration. The purpose of the experiments was to determine if the cooling and depressurization of coal-leachant slurry after hydrothermal leaching treatment results in precipitation, on product coal, of species containing sulfur, sodium, and ash that were soluble at the conditions of the hydrothermal leaching treatment. In the experiments, the coal-leachant slurry was filtered at 250.degree. C and 600 psi. The resulting coal was washed three times at 250.degree. C, carrying out pressure filtration between washes.
As shown by data presented hereinafter, the sulfur, ash, and the sodium contents were significantly lower, when pressure filtration was used, by comparison with the results of a standard leaching experiment.
The above results explain why the ash, the sodium, and, quite often, the sulfur contents of the product coal have been observed to increase during slow cooling of the product slurry.